Tiny typing fix

The Regions class is implicitly iterable because it defines __len__ and
__getitem__.
Mypy doesn't like this, and the recommendation seems to be to just make __iter__
explicit: https://stackoverflow.com/a/61739436/13220684

cle/backends/regions.py

@@ -1,4 +1,4 @@
-from typing import Generic, List, Optional, TypeVar
+from typing import Generic, Iterator, List, Optional, TypeVar
 
 from cle.utils import key_bisect_find, key_bisect_insort_left
 
@@ -57,6 +57,9 @@ def __setitem__(self, idx: int, item: R) -> None:
         # update self._sorted_list
         self._sorted_list = self._make_sorted(self._list)
 
+    def __iter__(self) -> Iterator[R]:
+        return iter(self._list)
+
     def __len__(self) -> int:
         return len(self._list)